if he's 26 he probably doesn't have enough stamina to do what you're asking. don't spur and whip him he will just shut down. if he's not wanting to do something it's because he feels there is no point to doing it or he gets no reward
If you haven't, get him out of the arena. Take a road ride or trial ride. If u let him get away with it, he will keep doing it. If you wack and spur him he will dig in harder and lose respect for you. Change things up for him. I would also check for soreness or injury, teeth, etc. My personal opinion.
Make sure that your horse is feeling well, sometimes stubbornness can come from unsoundness. Isn't always the case but it is definitely something to keep in mind!
Then keep that up. It's slow sometimes but any progress is good progress.
I have tried a crop and spurs and he gets even more stubborn. He is 26 and he was started very well but he was sold because he wasn't working for what they needed and then he just sat for a few years. Lunging does not work for him, he isn't afraid of the whip so he doesn't really know how to lunge.. the only thing that works is loving on him and gently asking him..
Push his timer little by little. Reward the extra time. My mare does this too and this method works like a charm now she's got stamina like no tomorrow.
Get some weapons and every time he stops turn his head to your knee and spur and whip him until he wants to go forwards.
Sounds like this horse was started poorly or has learned he can get away with things. Absolutely go back to groundwork and get him responsive to you. Lunge, lunge, lunge until he doesn't want to lunge anymore. Then lunge him harder. It sounds harsh but he needs to learn you're the boss, not him. Lots of yielding and flexing. Use a plastic bag if you can't seem to keep him going while lunging.
Try doing something that is fun for him, that he enjoys doing.
I rode one like this growing up. I would bug the crap out of him with a crop... i would tap tap tap until he moved. If he wouldn't move forward, I would bend his head and tap tap tap again. I would pester him until I got even the smallest movement. Even if he only took one step, I would stop and praise him and start again. If you tap him with a crop over and over and don't get a reaction keep doing it a little harder until you get a response.
I would use a crop or spurs